1. Technical Field
The present invention relates to a method and a device for embedding and detecting additional watermarking information in a black and white binary document image, said method and device embedding such information in the form not immediately recognizable to human eyes so as to facilitate prevention of cheating or protection of various rights.
2. Prior Art
Along with the globalization of business activities and the increasing awareness of consumers' rights, increasingly strict control has been required as to information handled by companies and public institutions. It can be considered as a consequence of the established consensus that wrongful spilling, forgery or tampering of information are disadvantageous to society in many ways in the forms of obstruction to fair competition or invasion of privacy and the like. Although information is mostly managed by a computer nowadays, spilling and leakage of information are often conducted using paper. While paperless work is advocated consumption of paper in offices is recently increasing in the form of printout from computers. Furthermore, diffusion of the office appliances such as copiers and faxes brought about a situation in which spilling of information can easily occur. Under such circumstances, a technical means to prevent paper-based spilling and leakage of information and to trace any spilled and leaked document is required.
Moreover, while diffusion of the DTP (desk top publishing) software and printers has facilitated creation of high-quality print documents, it has also increased the risk that a document which is seemingly true other than the different contents from the original may be created, namely forged and abused. To indicate that a document is not forged, a special form or ink, such as a form with a physical watermark can be used, but it leads to higher running costs. However, general documents are those printed and copied using ordinary forms and ink, and a method whereby information ensuring their authenticity can be attached to them is required.
For instance, Japanese Unexamined Patent Publication No. Hei7-84485, abandoned without request for examination, discloses a technique for embedding a watermark to identify an output device of a copier, and this technique implements embedding by changing brightness of the yellow toner in two or more areas placed on the entire page. As a matter of course, scanning of a colored and multivalued image is required to detect it, but documents are generally not multivalued but black and white binary, and it is not practical cost-wise to go to the extent of adding a function for capturing a color image to a black and white copier or a fax in order to detect a watermark.
Japanese Unexamined Patent Publication No. Hei6-324625, abandoned without request for examination, discloses a technique for embedding a watermark by means of subtle differences in shape such as a touch of a character. However, an image scanned after printing on paper has changed from the original image at a pixel level since it is influenced by the type of a printer's printing mechanism and the difference in resolution between a printer and a scanner. In addition, there are also changes due to establishment of density and how the document is written such as misregistration or skew (slant of paper) on copying, printing and scanning, and an effect of noise such as stains and blurs. It is difficult to stably detect, among these changes, a local change in shape due to embedding.
Japanese Unexamined Patent Publication No. Hei7-222000, now Japanese Patent 3,136,061, discloses a technique for embedding a watermark by increasing and decreasing vertical intervals of a center line in text lines. This technique can endure the scan after printing, but it cannot be applied to the above-mentioned scenario for preventing spilling and leakage since it requires the information extracted from the original document on detection.
An object of the present invention is to solve the above-mentioned problem and provide a method and a device for embedding additional watermarking information even into the data that is usually a black and white binary document image on paper and stably detecting the embedded information.